The present invention relates to a spectroscopy procedure, which is especially suitable for the near-infrared (IR). From the U.S. Pat. No. 4,158,505 a spectroscopy procedure is known, which uses a detector made of a photoconductive semiconductor material. With this procedure, the radiation deriving from a sample is limited like a channel, analyzed in a spectral manner and the detector is illuminated with the spectrum received in the above mentioned way and by using the dependency of the conductivity of the semiconductor material evaluated by the intensity of radiation, to produce phases of brightness and phases of darkness, the path of the rays is periodically interrupted between the sample and the detector via a chopper, at least one measurement cycle is performed and the measured values of a quantity to be measured, being clearly dependent of the conductivity of the detector, are determined during the phase of darkness as well as during the phase of brightness.
It is known that photoconductive semiconductors are used as detector material in spectrometers by using the dependency of their conductivity value of the incident beams. Thereby such a detector (photoresistor) is generally exposed to a direct voltage in combination with a capacitor, whereby then the capacitor is charged to a certain level depending on the intensity of the incident beams and the gate time. The voltage which can be calipered at the capacitor after the gate time represents the quantity to be measured.
The infrared spectroscopy is used preferably with the detection and quantitative determination of components in gaseous, liquid and solid material, whereby the interaction of the used IR-radiation with the colloidal movement of the molecules of the desired components is used for the analysis. This kind of interaction is at a maximum at a spectral range of about 1.5 .mu.m to 2.5 .mu.m, as the respective frequency of radiation coincides with the typical frequency of the combined vibration and the first overtone vibrations.
At the present time, as covering this spectral range, photoconductive semiconductor materials PbS and PbSe are known. When using such photoresistors in the spectroscopy, problems may occur due to two reasons, especially. On one hand the conductivity of such materials is depending on the temperature in a sensitive way. That is why the photoresistors are generally kept at a fairly constant temperature of slightly under 0.degree. C. for example via Peltier cooling, whereby a temperature drift at the detector element cannot be excluded, however. On the other hand, the charge carriers, produced by the radiation, cannot be recombined by a simple short-circuiting of the photoresistors, that is why the photoresistors possess a ,,memory" for the state of the pre-exposure. From the latter it can be concluded that the same luminous intensity can produce different conductivity values depending on the state of the pre-exposure with the same detector.
To avoid the influence of the state of the pre-exposure, the radiation, hitting a photoconductive detector of the semiconductor, can be interrupted in a periodic electronic or mechanic way, so that alternately phases of brightness and phases of darkness occur. The phase of darkness serves hereby the purpose of a regular production of a reference ground level state.
It is known that for the periodical interruption of the radiation, mechanical choppers with a constant frequency of rotation are used. At the same time with the periodic interruption of the radiation, the detectors are selected with the known procedure with a frequency, which is higher by a multiple. Thereby a number of measurements within a phase of darkness is received, a further number received in the interim state between the phase of darkness and the phase of brightness as well as a number of measurements in the phase of brightness and so on, so that given several rotations of the chopper, the result is an undulated profile of the measured values. By comparing the quantities to be measured in the phase of brightness with those in the phase of darkness, the intensity of the radiation, which meets the detector, can be determined.
This procedure has the disadvantage especially in that a measurement over several phases of brightness and phases of darkness has to be performed for the determination of the desired quantity to be measured. The accuracy of measurement is increased by the number of measurements performed. That way the speed of the known procedure is limited because of the high amount of measurements for the determination of the intensity of the radiation. Changes of the intensity of the radiation within a time frame of several individual measurements cannot be detected accordingly.